1. Field of the Invention
The principles of the present invention generally relate to liquid crystal display (LCD) devices. More particularly, the principles of the present invention relate to a fluorescent lamp driving unit and a method for driving the same, wherein the fluorescent lamp driving unit is capable of independently driving individual fluorescent lamps within a backlight unit.
2. Discussion of the Related Art
As information communication technology continues to develop, display devices become more important. Traditionally, cathode ray tubes (CRTs) have been used as display devices due to their ability to display color images at a high brightness. Compared to other, more recently developed types of flat display devices however, CRTs are relatively large and heavy. Therefore, many applications substitute CRTs for flat panel displays (e.g., liquid crystal display (LCD) devices, electroluminescent display (ELD) devices, plasma display panels (PDPs), etc.) that have large display areas, slim profile, high resolution, and are lightweight. Such flat panel displays have been developed for use as monitors for computers, spacecraft, and aircraft.
Due to their ability to efficiently display bright, moving images at high resolutions using relatively low driving voltages (and thus low power consumption) LCD devices are extensively researched and implemented in various applications.
A typical LCD device includes an LCD panel that display images by manipulating anisotropic optical characteristics of liquid crystal material contained therein. The optical characteristics of liquid crystal material are voltage-dependent. Accordingly, when predetermined voltages are applied to liquid crystal material of individual pixels, the polarization characteristics of each pixel are manipulated so as to transmit a predetermined of light that is incident to the LCD panel, thereby displaying an image. By themselves, LCD panels do not generate light that is necessary to display images. Therefore, to display images, light must be generated by a light source that is external to the LCD panel. Depending upon the light source used to display images, LCD devices may generally be classified as being either reflective- or transmissive-type LCD devices.
Reflective-type LCD devices use ambient light as a light source but have several drawbacks as the brightness of the images displayed depends on the brightness of light in the surrounding environment. Transmissive-type LCD devices, however, incorporate backlight units which contain a light source (e.g., electro-luminescent (EL) source, light-emitting diode (LED), cold cathode fluorescent lamp (CCFL), hot cathode fluorescent lamp (HCFL), etc.). Due to their thin profile and low power consumption, CCFLs are widely used as light sources in backlight units.
If AC power is directly applied to a plurality of CCFLs connected in parallel, only some of the CCFLs will be driven at one time. Thus, and to simultaneously drive the plurality of CCFLs connected in parallel, each CCFL must undesirably be connected to its own inverter (i.e., a power source). To overcome the disadvantageous use of CCFLs within backlight units, backlight units may be provided with external electrode fluorescent lamps (EEFLs) as the light source, wherein such backlights generally include a plurality of EEFLs connected in parallel. Contrary to CCFLs, a plurality of EEFLs connected in parallel may be driven using a single inverter (i.e., power source)
FIG. 1 illustrates a block diagram of a related art LCD device.
Referring to FIG. 1, a related art LCD device includes an LCD panel 11, a data driver 11b, a gate driver 11a, a timing controller 13, a power source 14, a gamma reference voltage part 15, a DC/DC converter 16, a backlight 18, and an inverter 19. The LCD panel 11 displays images and includes a thin film transistor (TFT) array substrate, a color filer array substrate, and a liquid crystal layer between the TFT and color filter array substrates. The TFT array substrate includes a plurality of gate lines G and a plurality of data lines D while the color filter array substrate includes a color filter layer. The data driver 11b supplies data signals to each data line D and the gate driver 11a supplies scanning pulses to each gate line G. The timing controller 13 receives graphic information (e.g., R, G, and B data), vertical and horizontal synchronizing signals Vsync and Hsync, a clock signal DCLK, and a control signal DTEN output by a liquid crystal module (LCM) driving system 17. The timing controller 13 also formats the received display data, the clock and control signals at a predetermined timing value to drive the gate driver 11a and the data driver 11b to effect the display of images. The power source 14 supplies a voltage to the timing controller 13, the data driver 11b, the gate driver 11a, the gamma reference voltage part 15, and the DC/DC converter 16. The gamma reference voltage part 15 receives the voltage supplied by the power source 14 and generates suitable reference voltages corresponding to analog data output by the data driver 11b, wherein the analog data is generated in association with the digital data output by the timing controller 13. The DC/DC converter 16 receives the voltage supplied by the power source 14 and generates a constant voltage VDD, a gate high voltage VGH, a gate low voltage VGL, a reference voltage Vref, and a common voltage Vcom to various components of the LCD panel 11. The backlight unit 18 includes a light source for emitting light to the LCD panel 11 and the inverter 19 drives the backlight unit 18.
A more detailed description of the backlight unit 18 and the inverter 19 will now be provided with respect to FIG. 2, illustrating a circuit diagram of a related art inverter used in driving a fluorescent lamp.
Referring to FIG. 2, the related art inverter includes a transformer T1, a high-frequency oscillation circuit 25, a first transistor Q1, a pulse width modulation (PWM) controller 24, and a power switch 26. The transformer T1 is connected to one end of a fluorescent lamp 10 included within the backlight unit 18 while the high-frequency oscillation circuit 25 is connected to a primary coil L1 of the transformer T1. The first transistor Q1 is connected between the high-frequency oscillation circuit 25 and a voltage source Vin such that the first transistor Q1 transmits a voltage output by the voltage source Vin to the high-frequency oscillation circuit 25. The PWM controller 24 supplies a control signal to the first transistor Q1 while the power switch 26 is connected between the PWM controller 24 and the voltage source Vin.
The transformer T1 includes the primary coil L1, a secondary coil L2, and an auxiliary coil L3. The primary and auxiliary coils L1 and L3, respectively, are connected to the high-frequency oscillation circuit 25. Accordingly, a first end of the secondary coil L2 is connected to the end of the fluorescent lamp, generically referred to at reference numeral 10, via the first capacitor C1 and a second end of the secondary coil L2 is connected to a grounding voltage source GND.
The high-frequency oscillation circuit 25 includes second and third transistors Q2 and Q3, respectively, and a second capacitor C2 connected in parallel to the primary coil L1, wherein the second and third transistors Q2 and Q3 are n-type and p-type transistors, respectively. The grounding voltage source GND is provided between the second and third transistors Q2 and Q3 and the second and third transistors Q2 and Q3 apply the voltage to the primary coil L1 according to the inputted AC voltage.
Collector terminals of the second and third transistors Q2 and Q3 are connected to opposing ends of the primary coil L1, emitter terminals of the second and third transistors Q2 and Q3 are commonly connected to the grounding voltage source GND, and base terminals of the second and third transistors Q2 and Q3 contact the central point of the primary coil L1 via first and second resistances R1 and R2.
Furthermore, a coil is connected between the collector terminal of the first transistor Q1 and the high-frequency oscillation circuit 25 while a first diode D1 is connected between the collector terminal of the first transistor Q1 and the grounding voltage source GND. Moreover, a synchronizing signal controller 28 is provided between the PWM controller 24 and a first node N1, wherein the first node N1 is formed between the coil and the first transistor Q1.
Upon activating the power switch 26, the PWM controller 24 receives a feedback current FB from the fluorescent lamp 10 and supplies a predetermined PWM control signal to the base terminal of the first transistor Q1. At this time, the PWM control signal controls a switching period of the first transistor Q1 according to the feedback current FB.
The first transistor Q1 is turned on and off in accordance with the PWM control signal output by the PWM controller 24. Accordingly, a voltage provided from the voltage source Vin and having a pulse width modulated by the PWM control signal is supplied to the high-frequency oscillation circuit 25. The coil removes the noise from the voltage transmitted by the first transistor Q1 and the first diode D1 prevents the voltage from flowing to the grounding voltage source GND. The synchronizing signal controller 28 receives the voltage signal having the noise removed by feedback and, in turn, generates a synchronizing signal for determining an output point of the PWM control signal outputted from the PWM controller 24. The synchronizing signal controller 28 then outputs the synchronizing signal to the PWM controller 24.
A detailed description of a first related art fluorescent lamp driving unit will now be provided with respect to FIG. 3.
Referring to FIG. 3, a related art fluorescent lamp driving unit includes a plurality of fluorescent lamps, herein provided as CCFLs 31, and a plurality of the aforementioned inverters 19. The plurality of CCFLs 31 are spaced apart from each other within the backlight unit 18 at a fixed distance to uniformly emit light. Moreover, each of the plurality of inverters 19 are connected with corresponding ones of the CCFLs 31 to apply driving signals to individual ones of the CCFLs 31, thereby individually driving corresponding ones of the CCFLs 31. Referring to FIG. 4, electrodes 33 are formed at opposing ends of each CCFL 31. Accordingly, each of the plurality of inverters 19 are connected with the electrodes 33 of each CCFL 31, enabling each CCFL 31 to be independently driven as desired. As discussed above, CCFLs 31 within the aforementioned backlight unit 18 can only be simultaneously driven when they are connected to their own inverter 19. However, driving each CCFL 31 using a unique inverter 19 can undesirably increase the cost of fabricating and maintaining the related art fluorescent lamp driving unit shown in FIG. 3 as the number of CCFLs contained within the backlight unit 18 increases.
Thus, and with reference to FIG. 5, a second related art fluorescent lamp driving unit replaces the CCFLs 31 with EEFLs 41. As shown in FIG. 5, a plurality of EEFLs 41 are spaced apart from each other within the backlight unit 18 at a predetermined distance. Because common external electrodes 42 (i.e., external electrodes of adjacent EEFLs 41 that are electrically connected to each other) are formed at both ends of each EEFL 41, the EEFLs 41 can be connected to each other in parallel and be driven using only one inverter 19. Accordingly, one inverter 19 can be used to simultaneously drive each EEFL 41. Referring to FIG. 6 common external electrodes 42 cover both ends of the EEFLs 41 and one inverter 19 is connected with the common external electrodes 42 of the EEFLs 41, to simultaneously drive the plurality of EEFLs 41.
When used in applications such as televisions, it is generally known that liquid crystal material within LCD devices can have a relatively slow response time, resulting in a blurring phenomenon of moving images. To overcome this disadvantage, driving techniques such as overdriving, and backlight modulation techniques such as flashing, data blinking, and scanning, have been developed. According to the overdriving method, data signals having higher values than preset data signals are applied to mitigate the effects of a slow response time of the liquid crystal material. According to the flashing method, the backlight unit is turned on and off in each frame to emulate the impulsive characteristics of CRTs. According to the scanning method, the backlight unit is turned on and off in synchrony with the application of a gate signal in one frame. Because the EEFLs 41 in the related art fluorescent lamp driving unit shown in FIG. 5 are driven using the same inverter 19, it is impossible to apply the aforementioned backlight modulation techniques.